Variable frequency drives (VFD's) for electric motor-driven centrifugal pumps are commercially available today. They are effective in providing a significant improvement over the efficiency of previously available constant speed electric motor-driven centrifugal pumps. These pump motor drives provide an effective “flow compensation” feature that results in energy savings by varying the electric current frequency rate powering the motor, to directly modify the output of the pump driven by the motor, as demand may change. Further improvements in efficiency are always welcome, and the present invention results in further improvement in the energy usage by the electric motor driving the pump, in real situations. The present invention is especially effective for electric motor-driven, centrifugal pumps subject to widely varying demand. In the past, such systems generally required a complicated initial set-up in order to carry out its control of the system in the most efficient manner. The most advanced such systems did not require external sensors in order to properly control the pump system, relying instead on internal data bases.
It is a well-known general rule of engineering to specify equipment providing a substantial margin of safety, such as specifying pumps and the motors driving them that are 50% to 100% larger than is necessary for the particular context, so as to avoid problems in any unexpected situation, or as the pump and motor age. However, the effect of that built in margin of error on the efficiency of the system, in regards to energy usage, was not fully appreciated in the context of the variable frequency drive systems, or at least they did not include means to compensate for such margin. Previously, in order to reduce the maximum flow, it was necessary to change the system, by introducing a valve at the outlet from the pump, calibrated by extensive and costly testing, in addition to the internal data base of the VFD. The prior system also included an algorithm to maintain the pump at a constant flow output, regardless of the change in pump head resistance.
Examples of the operation of a VFD are widely available, for example from the Danish company Danfoss Low Power Drives, in U.S. Pat. Nos. 8,564,233 and 8,436,559, as well as U.S. Pat. Nos. 6,715,996 and 6,354,805, describing systems without external sensors. Systems commercially available include the Danfoss VLT HVAC Drive.
Such controller systems include software executed by a digital signal processor (DSP) or a microprocessor, and can perform real-time control by using the input electric current and the power of the electric motor as control variable, to the effect that the desired operating point of the motor, and hence of the pump, remains stable. The current frequency and the electrical power input are compared with an empirically prepared, stored table of associated input power and electrical frequency, compared to liquid flow and pressure head. As a further enhancement, to compensate for any effect the heating of the motor may have on power output, the data can also include any power changes of the motor from the operating temperature of the motor.